A metallic circuit board generally consists of a metallic plate made of aluminium, steel or other metallic material, an insulating layer placed thereon, and a printed circuit pattern formed on the insulating layer. Because such a metallic circuit board can be used as a structural member, it can contribute to the reduction in the size of the equipment or the system into which it is incorporated. For instance, a metallic circuit board is widely used as a part of the casing of a floppy disk drive unit for the purpose of reducing its thickness. In such applications, countersunk holes are formed in the metallic circuit board so that countersunk head screws may be used for securing the metallic printed circuit board to other parts of the equipment without involving any protrusions from the surface of the metallic printed circuit board.
Conventionally, such countersunk holes have been formed by using chamfering cutters and drill bits, but such a machining process produces metallic chips which are required to be thoroughly removed for the metallic printed circuit board to be safely used in electronic equipment, and this significantly impairs the manufacturing efficiency. Further, such machining work requires cutting oils and compounds, and they also must be thoroughly removed.
Alternatively, it is conceivable to use a cylindrical punch having a conical or otherwise tapered end surface so as to form a countersunk hole in cooperation with a suitable die. Such a punching process requires less time and incurs less cost than a machining process. However, the pressure from the punch causes the material of the metallic plate in the immediate vicinity of the hole to flow radially inwardly, and the resulting relative shifting between the metallic base plate and the insulating layer tends to cause peeling of the insulating layer from the metallic base plate.